As far as the sub-millimeter range is concerned, terahertz technology been primarily been used in the fields of terrestrial astronomy and earth observation. However, many materials that are opaque in the optical and infrared regions are transparent to terahertz waves (0.1 THz to 10 THz). Applications for terahertz technology have thus recently expanded to include areas such as aerial navigation where terahertz waves are able to penetrate clouds and fog, medical imaging where body tissue can be examined without using potentially harmful ionizing radiation, and non-invasive security systems for use at airports and ports in which the terahertz waves are able to pass through clothing and materials normally opaque to infrared.
Due to the sub-millimeter wavelengths of terahertz waves, the required dimensions and accuracy of components such as antennas, waveguides, lenses, mirrors etc. make fabrication difficult and costly using conventional manufacturing techniques.
In the millimeter waveband, ferroelectric phase shifters are often employed in which the phase of the signal is shifted by varying the permittivity of the ferroelectric material by means of an applied electric field. However, ferroelectric phase shifters suffer from substantial power losses, signal distortions and noise, and offer only discrete steps.
An optically activated waveguide type phase shifter and/or attenuator has been disclosed in U.S. Pat. No. 5,099,214 (ROSEN et al.). This device comprises a semiconductor slab that is attached to an inside wall of a waveguide and which receives light from an illumination source disposed in an aperture of an inside wall opposite inside wall. In U.S. Pat. No. 4,263,570 (DE FONZO), a piece of semiconductor material is attached to an inside wall of a waveguide and an inside surface of said piece is lit from outside by a light source through an aperture in a wall opposite inside wall.
In these prior art documents, where illumination is from the opposite waveguide wall, a lossy resistive layer forms inside the waveguide at a distance from the inside wall that is equal to the thickness of the semiconductor piece or slab, which means that the insertion losses will be always high, and that a high level of light is necessary to obtain a significant phase shift or attenuation. Namely, this light level should be generally high enough to generate a high density of carriers to place the photo-sensitive material (Si) in a metallic or semi-metallic state.
It is therefore an object of the present invention to provide a tuneable phase shifter and/or attenuator capable of operating at microwave, millimetric and/or sub-millimetric wavelengths with an improved tuneability. According to the invention, this is obtained by a positioning of a light source and/or a photo-responsive material spaced relatively to the waveguide, and by providing a modification of the carrier concentration within a photo-responsive material by the illumination of light.